1. Field of the Invention
The present invention relates to a digital microphone in which a digital signal output can be directly obtained by a method completely different from that used in conventional digital microphones, and which is preferably used for professional use.
2. Description of the Related Art
A professional-use audio microphone should fulfill a broad frequency band equal to or broader than 100 KHz, and extremely wide dynamic range of 140 dB. These required specifications are difficult to be achieved with the current digital technology. The present invention relates to a digital microphone whereby the above-mentioned required specifications can be achieved with a unique idea. Before proceeding to a description of the present invention, the conventional related arts will now be described.
A conventional digital output microphone mainly comprises a microphone unit, a preamplifier, and an analog/digital converter (hereinafter referred to as “AD converter”). The microphone unit performs electroacoustic conversion after receiving a sound wave, and output an audio signal as an analog signal. The output audio signal, after being amplified by the preamplifier, is converted into a digital signal by the AD converter, and then is output as a microphone output.
FIG. 4 shows an exemplary conventional digital output microphone. Referring to FIG. 4, a microphone 80 is, for example, a condenser microphone. When a diaphragm (hereinafter referred to as “membrane”) of the condenser microphone vibrates by receiving a sound wave, the capacitance between the membrane and a fixed electrode opposite to the membrane is changed. The change in the capacitance causes a voltage change which is then amplified by a preamplifier 82 and is impedance-conversed, and is input to an AD converter to be converted into a digital signal. In this conventional digital microphone, the AD converter comprises a delta-sigma (hereinafter referred to as “ΔΣ”) modulator 84 functioning in synchronization with a high-frequency clock and is converting an analog input signal into a one-bit density modulated digital signal, and a digital filter 86 functioning in synchronization with a low-frequency clock.
The above-mentioned ΔΣ modulator 84 is a core of the AD converter, and as shown in FIG. 5, comprises an integrator 842, a one-bit quantizer 843 embodied by a comparator or the like, and a digital/analog converter (hereinafter referred to as “DA converter”) 845. The DA converter 845 serves as a unit for providing a negative-feedback of an output from the one-bit quantizer 843. In the ΔΣ modulator 84 configured as mentioned above, an input analog signal is sampled at an extremely higher frequency than the audio signal band (i.e., oversampling), and is converted into a one-bit pulse density modulated digital signal. As a result, the noise shaping effect can be brought about in which quantization noises are moved to the high-frequency side.
A digital microphone with use of a ΔΣ modulator has been already known. Examples thereof include a digital microphone disclosed in Patent Document 1. Patent Document 1 discloses a digital microphone comprising a transducer for generating an acoustic signal of an analog signal corresponding to a sound wave, and a single bit ΔΣ modulator AD converter for generating a digital output signal from the analog signal in the form of a ΔΣ modulated bit stream at an oversampled rate. The digital microphone of Patent Document 1 is basically identical to the conventional digital microphone as shown in FIGS. 4 and 5.
[Patent Document 1] Published Japanese Translation of PCT International Publication for Patent Application No. 2005-519547